1. Field of the Invention
The present invention relates to a motor, and more particularly, to a motor having a reverse-rotation preventing unit capable of preventing a reverse rotation of a rotor at the time of an initial driving, simplifying a structure, and being easily fabricated.
2. Description of the Background Art
Generally, a motor for converting electric energy into kinetic energy is applied to various fields such as home appliances, etc., and is used as a driving source of the home appliances, etc. For example, when the motor is applied to a refrigerator, the motor rotates a fan thus to circulate cool air inside the refrigerator. When the motor is applied to an air conditioner, the motor rotates a fan thus to flow cool air generated from an evaporator to an indoor room. There are many kinds of motors according to an application field.
As a kind of the motor, an induction motor includes a stator for forming a rotating magnet field, and an induction rotor rotatably inserted into the stator. Recently, an induction motor having a permanent magnet between the stator and the induction rotor for the efficiency enhancement is being developed.
FIG. 1 is a frontal view showing an induction motor in accordance with the conventional art, and FIG. 2 is a sectional view showing an induction rotor of the induction motor.
As shown, the induction motor comprises a stator 100 having winding coils thereon, an induction rotor 200 rotatably inserted into the stator 100, and a synchronous rotor 300 rotatably inserted between the stator 100 and the induction rotor 200.
The stator 100 comprises a stator core 110 having a certain length, and winding coils 120 wound on a plurality of teeth 111 formed in the stator core 110 and generating a rotating magnet field. The stator core 110 is a lamination body formed accordingly as a plurality of sheets are laminated.
The induction rotor 200 comprises a rotor core 210 of a filled cylindrical shape having a certain length and an outer diameter, and a cage 220 inserted into the rotor core 210. A rotation shaft 230 is coupled to a center of the rotor core 210. The rotor core 210 is a lamination body formed accordingly as a plurality of sheets are laminated. The cage 220 includes a ring-shaped end ring 221 positioned at both side surfaces of the rotor core 210, and a plurality of connection rods 222 positioned in the rotor core 210 and connecting the two end rings 221. The cage 220 is a conductor, and is formed at the rotor core 210 by an insert-molding method.
The induction rotor 200 is inserted into an insertion hole of the stator 100.
The synchronous rotor 300 comprises a permanent magnet 310 of a hollow cylindrical type having a certain thickness, and a holder 320 having a cup shape for supporting the permanent magnet 310. The permanent magnet 310 is rotatably inserted into an air gap between the stator 100 and the induction rotor 200. A bearing 330 is coupled to one side of the holder 320, and the bearing 330 is coupled to a rotation shaft 230.
The stator 100 is mounted in a motor casing 400, and bearings 410 are provided at both side surfaces of the motor casing 400. The rotation shaft 230 is coupled to the bearings 410.
In the induction motor, a rotation force is transmitted to a load through the rotation shaft 230, and a fan 240 is mounted at the rotation shaft 230.
An operation of the induction motor will be explained as follows.
When power is supplied to the stator 100 and a rotating magnet field is formed by the applied power, the synchronous rotor 300 having the permanent magnet 310 is relatively rotated centering around the rotation shaft 230. As the synchronous rotor 300 is rotated, an induction current flows to the cage 220 of the induction rotor 200 by a flux of the permanent magnet 310 of the synchronous rotor 300. Herein, the induction rotor 200 is rotated by the rotating magnet field of the stator 100, the permanent magnet 310 of the synchronous rotor 300, the induction current applied to the induction rotor 200, etc.
When the induction motor is initially driven, a rotation speed of the induction rotor 200 reaches up to a synchronous speed by the permanent magnet 310 of the synchronous rotor 300 and a current applied to a sub winding coil of the winding coil 120. Then, the induction rotor 200 is rotated by a current applied to a main winding coil of the winding coil 120.
However, in the induction motor, the synchronous rotor 300 and the induction rotor 200 may be reverse-rotated by an abnormal voltage phase and an uneven rotating magnet field generated when the motor is initially driven. The synchronous rotor 300 and the induction rotor 200 have a larger tendency to be reverse-rotated when a load inertia is smaller and a voltage is larger.
In order to prevent a reverse-rotation of the synchronous rotor 300 and the induction rotor 200 of the induction motor, a reverse-rotation preventing unit has been provided at the induction motor, However, the reverse-rotation preventing unit has a complicated structure and an expensive fabrication cost.
Techniques for preventing a reverse-rotation of the motor by a mechanical method have been disclosed in the U.S. Pat. No. 4,893,038 (1989.01.09), Japanese Examined Patent Publication No. 6-25624 (1994.04.06), Japanese Examined Utility Model Publication No. 8-11037 (1996.03.29), Japanese Patent Publication No. 9-163663 (1997.06.20), and Korean Open-Laid Publication No. 1998-0003324 (1998.03.30).
However, according to the techniques for preventing a reverse-rotation of the motor by a mechanical method, an entire structure is complicated and a reliability of the motor is low.